Effects of Oxidative and Alkaline Post-treatments on Removal of Trivalent Iron and Hexavalent Chromium Using Peanut Shell Biochars

Abstract

Objectives:The main goal of this study was to compare the removal efficiency of trivalent iron and hexavalent chromium by peanut shell biochars (i.e., PB), post-treated peanut shell biochars using KMnO4 (i.e., PB-Ox), and secondary post-treated peanut shell biochars using KOH (i.e., PB-Ox-A).Methods:The adsorption mechanisms of trivalent iron and hexavalent chromium by PB, PB-Ox, and PB-Ox-A were investigated using two types of adsorption kinetic and isotherm models. Furthermore, the adsorption experiments were performed under different adsorbent dosages (0.8 - 2.4 g/L), temperatures (15 - 35℃) and ion strengths (0.05 - 0.2 M NaNO<sub>3</sub>) to identify their effects on the adsorption of trivalent iron and hexavalent chromium by PB, PB-Ox, and PB-Ox-A.Results and Discussion:Trivalent iron and hexavalent chromium could be more effectively removed by PB-Ox-A than PB and PB-Ox because of its higher contents of oxygen containing functional groups (O/C of PB = 0.064; O/C of PB-Ox = 0.058; O/C of PB-Ox-A = 0.188), higher surface area (PB = 351.5 m<sup>2</sup>/g; PB-Ox = 344.0 m<sup>2</sup>/g; PB-Ox-A = 2121.5 m<sup>2</sup>/g), and greater pore volume (PB = 0.15 cm<sup>3</sup>/g; PB-Ox = 0.15 cm<sup>3</sup>/g; PB-Ox-A = 0.96 cm<sup>3</sup>/g). The removal efficiencies of trivalent iron and hexavalent chromium by PB, PB-Ox and PB-Ox-A were increased with increasing the adsorbent dosages (PB-Ox-A > PB-Ox > PB). The adsorption kinetic experiments demonstrated that the pseudo second order rate model was suitable for the removal of trivalent iron and hexavalent chromium by PB (R<sup>2</sup> of Fe<sup>3+</sup> = 0.99; R<sup>2</sup> of Cr<sup>6+</sup> = 0.99), PB-Ox (R<sup>2</sup> of Fe<sup>3+</sup> = 0.98; R<sup>2</sup> of Cr<sup>6+</sup> = 0.98), PB-Ox-A (R<sup>2</sup> of Fe<sup>3+</sup>+ = 0.99; R<sup>2</sup> of Cr<sup>6+</sup>+ = 0.99). Furthermore, the removal of trivalent iron and hexavalent chromium using PB, PB-Ox and PB-Ox-A was well fitted to the Freundlich isotherm absorption model (R<sup>2</sup> of Fe<sup>3+</sup> = 0.997 - 0.999; R<sup>2</sup> of Cr<sup>6+</sup> = 0.995 - 0.998). The changes of temperature did not show significant effects on the removal of trivalent iron and hexavalent chromium by PB, PB-Ox, and PB-Ox-A. The removal efficiency of trivalent iron by PB, PB-Ox and PB-Ox-A was not influenced by the ionic strength whereas the removal efficiency of hexavalent chromium by PB, PB-Ox and PB-Ox-A was considerably decreased with increasing the ionic strength. These observations are evident that PB-Ox-A is the most effective adsorbent for the removal of trivalent iron and hexavalent chromium.Conclusions:The proposed post-treatment procedures might improve the surface properties of peanut shell biochars intimately associated with the removal of trivalent iron and hexavalent chromium. The physicochemical properties of the heavy metals and the biochars were found to be key factors governing the adsorption mechanisms of trivalent iron and hexavalent chromium by PB, PB-Ox and PB-Ox-A.